M. Raja (University of Liverpool)
Over the past decade, Organic Electronics has continued to evolve steadily, particularly in the development of simple digital circuits [1]. Efforts have also begun in the development of analog counterpart, for use as key functional blocks in flexible low-cost smart sensor systems [2 - 4]. This however has been generally challenging due to the complexity of the fabrication processes [5], owing to the circuit architecture adapted. In addition, the gain and bandwidth of the amplifier are low, attributed to the low charge carrier mobility of the active layer. Moreover, for solution-processed layers, the possible variability of the parameters can hinder the overall circuit performance [6 - 11]. In this talk, we propose novel approaches in the design of an organic analog circuit such as the Operational amplifier (Op-amp), for integration in a smart sensor technology. We propose methods to tackle and investigate the various issues addressed above. In order to maintain simpler processes, the design of the organic Op-amp adapts a pseudo-PMOS configuration with saturated loads, rather than the alternative CMOS. With such a configuration, only p-channel organic thin-film transistors (OTFTs) are utilised of higher charge carrier mobility, and subsequently require one type of metal contact for the source/drain of the load and driver OTFTs. And with saturated loads, smaller number of input gates is needed compared to the CMOS design [12] for better control of the threshold voltages of the load and driver OTFTs. There are however performance trade-offs to be considered when adapting the pseudo-PMOS designs, particularly in the voltage range for the logic levels, switching point and operational speeds. Nonetheless, such trade-offs can be compensated by adjusted the aspect ratios of the load to driver OTFTs, and utilising additional circuitry so as to boost the gain and attain adequate bandwidth for sensor applications. The Op-amp circuit designs are simulated using appropriate organic parameter values to reflect the conductance and capacitances of the OTFT, including low mobility of the charge carriers and high contact resistances. These models are verified using experimental data of 6,13-bis(tri-isopropylsilylethynyl) pentacene (TIPS) OTFT [13]. The effect of the variability of the parameters such mobility and threshold voltage, on the overall circuit performance are presented, and finally the functionality of the organic Op-Amp in sensing applications is demonstrated.
References
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[12] K. Myny et. Al., IEEE Journal of Solid State Circuits, 47(1), 2012.
[13] M. Raja et. al., Journal of Applied Physics, 112, 084503, 2012.
